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formula for pilot holes

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  • formula for pilot holes

    Is there a formula for drilling the correct size pilot hole?

  • #2
    I think the correct formula is not too small, not too big.

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    • #3
      Leave plenty of work for the following drill, small increments tend to eat flute tips.
      If you benefit from the Dunning-Kruger Effect you may not even know it ;-)

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      • #4
        = drill web diameter
        Location: Helsinki, Finland, Europe

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        • #5
          Thanks for the info guys I'm trying to drill 17/32 horizontal holes for tapping 5/8 X11 holes in HR rod, the 3/8 bit I used for drilling the pilot hole grabs the 17/32 bit and twists the drill motor in my hands. Both are new bits, what am I doing wrong?
          Last edited by tackit; 03-22-2018, 03:38 PM.

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          • #6
            Originally posted by tackit View Post
            Thanks for the info guys I'm trying to drill 7/32 horizontal holes for tapping 5/8 X11 holes in HR rod, the 3/8 bit I used for drilling the pilot hole grabs the 17/32 bit and twists the drill motor in my hands. Both are new bits, what am I doing wrong?
            Sounds like your pilot hole is too big and the 17/32 bit is actually trying to screw itself into the hole and binding up vs. drilling.

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            • #7
              Originally posted by 3 Phase Lightbulb View Post
              Sounds like your pilot hole is too big and the 17/32 bit is actually trying to screw itself into the hole and binding up vs. drilling.
              Yup. Without going to garage to look at my drills what would be typical web thickness on 13mm drill I'd say that the suitable pilot hole size is about 3/32" to 1/8" for the the 17/32" drill.
              3/8" pilot hole is suitable for 2" drill.
              Location: Helsinki, Finland, Europe

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              • #8
                I usually go with 1/4 to 1/3 drill diameter for a pilot.
                Peter - novice home machinist, modern motorcycle enthusiast.

                Denford Viceroy 280 Synchro (11 x 24)
                Herbert 0V adapted to R8 by 'Sir John'.
                Monarch 10EE 1942

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                • #9
                  OK, I'll do as you guys suggest from now on and drill smaller pilot holes. 1/4 of the hole size will be the easiest for me to remember.

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                  • #10
                    I was taught no bigger than 1/3 from a really smart guy that pops in here once in a while.. That
                    rule has served me very well... It hasn't served me well on the occasions that I've broken it.

                    In reality, your bigger drills are going to have a chisel point, and you really just need to clear
                    out a diameter that is bigger than the chisel point. That will drop your cutting forces massively,
                    and keep your hole closer to size. Even on bigger drills with a split point, I'll still pilot.

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                    • #11
                      As noted you really only want the pilot drill to be the width drill web. Drill manufacturers offer three and four flute core drills for drilling out holes to larger sizes. The nature of a three or four flute drill is it will self center in the existing hole.
                      Mike
                      Central Ohio, USA

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                      • #12
                        Originally posted by bobw53 View Post
                        I was taught no bigger than 1/3 from a really smart guy that pops in here once in a while.. That
                        rule has served me very well... It hasn't served me well on the occasions that I've broken it.

                        In reality, your bigger drills are going to have a chisel point, and you really just need to clear
                        out a diameter that is bigger than the chisel point. That will drop your cutting forces massively,
                        and keep your hole closer to size. Even on bigger drills with a split point, I'll still pilot.
                        Good info Bob , I have never heard your explanation before, but I understand it and will remember it...thanks.

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                        • #13
                          Drilling large holes with a hand drill is always a touchy thing. It is easy for the drill to grab if you don't hold it perfectly straight, especially one break through. Have a solid death grip on the drill with both hands and try to do it in a way you can still let go of the trigger instantly. Some set up the job so the cord is pulled tight in such a way if they start moving as when the drill starts to wind your wrist up it pulls the cord out of the wall.

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                          • #14
                            Originally posted by MattiJ View Post
                            = drill web diameter
                            That's the rule I follow.

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                            • #15
                              Yep, Drill web diameter.

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